The present invention relates to a dielectric resonator comprising a block of dielectric material, having upper, lower and side surfaces and in which there is a hole extending from the upper surface to the lower surface, the hole and the lower surface as well as at least part of the side surfaces being coated with an electrically conductive material, at least the upper surface being uncoated and the hole forming a transmission line resonator, and the uncoated surfaces are covered with a lid of an electrically conductive material, whereby the dielectric block is substantially surrounded by an electrically conductive material.
It is known that a dielectric resonator, for example, a ceramic resonator, comprises, in its basic structure, a block of dielectric material, for example, titanate, having a high dielectric constant, in which block a hole is made and which has side surfaces, as well as upper and lower surfaces and the hole extends from the upper surface of the block to the lower surface. The surfaces of the block are, with the exception of the upper surface, coated with an electrically conductive material. The hole, too, is coated with an electrically conductive material. The hole is short-circuited at the juncture where the coating of the coated hole joins the coating of the lower surface. Because the upper surface is uncoated at least in the vicinity of the hole, the hole is open at this end. The construction forms a power line resonator whose resonance frequency is determined by the length of the hole, that is, by the thickness of the dielectric block. The resonance frequency is formed in accordance with the equation ##EQU1## in which f.sub.R is the resonance frequency in Hertz, c is the velocity of light, .lambda. is the wavelength in meters and .epsilon..sub.r is the relative dielectric constant of the dielectric material. Accordingly, the resonance frequency in megahertz is formed roughly in accordance with the equation ##EQU2##
Usually the length of the hole is dimensioned in such a way as to yield a transmission line resonator a quarter wave in length. When an electromagnetic wave is introduced into the construction, a standing wave is produced in the direction of the hole at a given frequency, that is, the resonance frequency. The maximum of its capacitive field is at the open end of the hole, whereas the maximum of the inductive field is at the short-circuited end of the hole. If various conducting patterns are disposed in the uncoated upper surface, it is possible to exercise an effect on both the resonance frequency of an individual resonator and on the coupling between the resonators if there are several resonators. When more than one hole is formed in the dielectric block, that is, there is more than one transmission line resonator in parallel, a dielectric filter can be implemented which has several zero or pole points. By placing a conductor spot beside the open end of the outermost resonators of the block and such that it is insulated from the coating of the side of the block, a signal can be brought to the resonator by coupling it capacitatively to the resonator and it can be directed outward from the resonator with the same capacitive coupling. Because there is a specified capacitance value between the coating of the open upper end of the resonator and the coating of the upper edge of the side of the dielectric block, this capacitance can be changed by adding a coating to the upper side near the hole, the coating thus constituting a juncture with the coating of the side, or by adding a coating to the upper side, thus forming a juncture with the coating of the hole. This offers a way of affecting the resonance frequency. It is furthermore possible to make use of conducting patterns so as also to arrange on the upper surface--between the resonators--capacitors and transmission lines and thus to affect the coupling between the resonators. The inductive coupling between the resonators can be affected by treating the dielectric block, for example, by boring holes in it or otherwise by removing material from it.
Disposing conducting patterns on the upper surface of the dielectric block is nevertheless very troublesome because the available surface area is very small, which means that even small imprecisions in positioning the conductor patterns will have a great effect on the electrical characteristics of the filter. In addition, by positioning the conducting patterns solely on the upper surface, it is possible only to affect the capacitive field and the couplings are thus capacitive.
A decisive improvement in this generally used method is disclosed in the present applicant's patent application EP-0 401 839, Turunen et al. In the filter described therein, the electrical characteristics of the filter can be affected in a wide range such that the side surface of the dielectric block is substantially uncoated and the conductor patterns and coupling wires are disposed in this side surface of the filter block. Apart from the fact that a much more extensive surface area is now available for positioning the conducting patterns than when they are positioned on the upper surface, it is also possible to affect the inductive coupling between the resonators. The inductive field is indeed at its greatest at the short-circuited lower end of the resonator. Positioning the conductor pattern on the side surface thus permits making the connection between the resonators capacitive, inductive and capacitive-inductive in the same filter block. A coupling to the filter can also be made inductive, capacitive or a combination of these. Small variations in the positioning of the conductor patterns to the side of the block are not as sensitive in affecting the electrical properties of the filter as is the case when the patterns are positioned on the upper surface with its small surface area. According to the EP application, the side in which the conducting patterns are located is finally covered with a metal lid. This filter construction permits the filter designer a great latitude of freedom and in practice, using only a few standard-sized filter blocks, different types of filters can be constructed by varying the bandwidth and the average frequency of the resonators, that is, by using different kinds of conducting patterns.
The dielectric block is usually of ceramic material, which is pressed into a form and it can be very precisely fabricated to the correct size. There is nevertheless a need to tune the resonance frequency of the resonator. Particularly when filters are being formed, it is common to tune the resonance frequencies of the different resonators of the filter to different magnitudes depending on the characteristics which the filter is expected to provide.
One method of tuning the frequency of the resonator is to increase the capacitance at the open upper end of the resonator. By increasing the capacitance of the open end of the resonator, its resonance frequency can be reduced, whereby the resonator hole can also be fabricated so that it is shorter, thereby enabling the dielectric filter to be smaller in size. This capacitance can be implemented by means of an electrode plate positioned above the open end of the resonator, the plate thus forming a capacitance with the open end of the resonator. This kind of tuning element for the resonance frequency, which is based on the use of an electrode plate, can be implemented, for example, by means of an electrode plate 6a, 6b disposed at the end of an adjusting screw 7a, 7b mounted in enclosure 5, which covers the open end of the resonator, as is shown in FIG. 1, whereby by means of adjusting screw 7a, 7b the capacitance, that is, the distance between electrode plate 6a, 6b and the open end of resonator 3a, 3b, can be tuned. Another alternative for implementing this kind of resonance frequency tuning element is to form in enclosure 5, which is of an electrically conductive material, above the open end of the resonator, bent tabs 8a, 8b, as is shown in FIG. 2. The tabs 8a, 8b can be formed by cutting into enclosure 5, for example, U- or similarly shaped tabs. By bending these tabs 8a, 8b inwardly, that is, towards the resonator, the distance between the resonator and the tab is altered, in consequence of which the capacitance between the tab and the resonator and thus the resonance frequency of the resonator, changes. In FIGS. 1 and 2, reference number 1 shows a dielectric filter, reference number 2 shows a dielectric block and reference numbers 3a, 3b show holes formed in the dielectric block, which holes are coated with an electrically conductive material 4, forming the transmission line resonators. The lower surface and side surfaces of dielectric block 2 are also coated with an electrically conductive material, which joins the coating of resonator holes 3a, 3b. The upper surface 9 of the dielectric block is uncoated.
When a current travels in resonator 3a, a TEM wave is generated between the conductive layer surrounding the dielectric block, that is, coating 4 and enclosure 5, and resonator 3a, whereby TEM-modal electric, E, and magnetic, H, fields are formed in the dielectric block, as is shown in FIG. 3, which is a cross-section A-A' of FIG. 2, and in FIG. 4. The resonator acts as a kind of antenna and the component of the magnetic field of the TEM wave generates a modal wave, which oscillates strongly as the resonator 3b of the next stage. The electric and magnetic fields of this modal wave, couple resonators 3a and 3b to each other. In the resonator the orientation of the electrical field of the modal wave is from its lower end to the open upper end and the electrical field of this modal wave is the strongest inside the resonator tube at its upper end. As is shown in FIGS. 3 and 4, the electrical E and magnetic H fields do not radiate outwards from the dielectric block but remain in dielectric block 4 and in resonator tube 3a, 3b because the dielectric block binds the fields fairly strongly within itself owing to the high dielectric coefficient .epsilon..sub.r of the dielectric substance. Because the electrical field that is set up outside the resonator is thus weak, electrode 6a, 6b or tab 8a, 8b, which are positioned above the open end, do not provide a strong coupling or a very great frequency tuning effect.
For tuning the frequency of a resonator according to the prior art, the use of a so-called tuning plug is known, whereby a sleeve of electrically insulating material is disposed inside the resonator tube (3a, 3b in FIG. 2), inside of which sleeve an electrical conductor, for example, electrical wire, of a specified length is disposed, which is grounded at its upper end to the enclosure covering the upper surface of the resonator. In this manner the frequency can be tuned more effectively when the conductor that is connected to the ground plane is introduced into the resonator tube, in which the electrical field is stronger. Frequency tuning tab 8a, 8b and electrode plate 6a, 6b are of a form and size such that they do not fit inside resonator tube 3a, 3b, or bending the tab to make it go inside the resonator tube would at least be a very difficult and precision work stage to carry out if the tab were made to be so small in size that it would fit into the resonator hole.